The present invention relates generally to TDMA/TDD (Time Division Multiple Access / Time Division Duplex) radio communication systems, and especially to adaptation of the systems to prevailing radio conditions.
ETSI BRAN (Broadband Radio Access Network) is developing a short-range high data rate system, HIPERLAN Type 2 (also called H/2), mainly for indoor operation. Some outdoor scenarios are also considered (campus areas, downtown city areas). The target areas are offices, conference halls, exhibition fairs, airports and home environments. The spectrum is unlicensed and thus several xe2x80x9coperatorsxe2x80x9d may use the same spectrum. The interference environment may change during operation due to for example new operators in the vicinity of the own network and it is then very difficult to predict what type of interference the system shall be able to handle. The large difference in radio propagation, i.e. LOS (Line Of Sight) and NLOS (No Line Of Sight), and interference environments in which the system be must be able to operate, puts strong requirements on the system that it is able to adapt to its current situation. In this type of systems, one radio cell might be exposed to larger interference than other radio cells. Just an adaptation per radio cell to handle this situation is referred to as xe2x80x9cradio cell adaptationxe2x80x9d. Furthermore, the mobile terminals (MTs) associated with a certain base station (BS) may have different reception qualities in their uplink and downlink respectively. Hence, in this case each MT might want to use different transmission parameters, e.g. code rate (protection level) and modulation alphabet, to be able to adjust its reception quality in the uplink and downlink. This adaptation could be performed per MT or per its individual connections. In the latter case differing traffic and QoS (Quality of Service) parameters have to be considered. For example, one MT could have a connection carrying video using a powerful FEC (Forward Error Correction) code, whereas a connection for file transfer uses a less strong FEC but with ARQ (Automatic ReQuest for retransmission) capabilities.
Typical reception quality measures are:
retransmission rate (PER, Packet Error Rate),
delay spread (time dispersion),
received signal strength (RSSI),
Signal-to-Interference Ratio (SIR)
Bit Error Rate (BER)
Combinations of these performance measures and others are also possible.
Usually link adaptation is divided into two groups: net rate adaptation and gross rate adaptation.
Net rate adaptation means that the incoming data rate is adjusted to fit into the assigned capacity so that the system can handle a certain link quality, i.e. the user has a fixed assigned capacity over the air, and if the radio quality is poor the incoming data rate is reduced and a more robust transmission mode is used. In case of a good connection a higher incoming data rate can be used.
In gross rate adaptation the incoming data rate is xe2x80x9cfixedxe2x80x9d, i.e. the radio system does not change its incoming traffic due to the radio conditions. Instead the radio system tries to sustain the incoming data rate and to counter the variations in link quality by assigning correspondingly varying capacity over the air interface. Thus, two MT with the same incoming data rate could have been assigned different capacity over the air interface based on their individual connection reception qualities. An extra function might be needed in this case to guarantee fair utilisation of the total available capacity.
Combination of net and gross rate adaptation is of course also possible.
The present situation with regard to adaptation to varying radio conditions in different radio communication standards may be summarised as follows:
HIPERLAN/2: No proposal exists on a protocol that handles the ability to make radio cell adaptation and/or link (per MT or per connection) adaptations. Still, the proposals on the physical layer allow different code rates and modulation alphabets (MPSK and MQAM signal constellations).
GPRS: The system applies net rate link adaptation (selects channel coding) per mobile terminal, see [1]. For downlink traffic the MT request channel coding via ARQ-ACK/NACK messages through the uplink. The BS is using stolen bits (embedded in the burst structure of GSM) to set the channel code for the downlink. Hence, the MT first decodes these bits to obtain information on which channel decoding it shall use for the rest of the burst. In case unacknowledged mode is applied, the MT sends measurements reports to the BS including an estimation of the BER. This information can then be used by the BS to select channel coding for the downlink bursts.
For the uplink traffic the BS commands the MT to use a certain channel coding. This information is transferred to the MT piggybacked on downlink dedicated control channels, e.g. piggybacked on ARQ-ACK/NACK messages.
A drawback is that in GRPS it is not possible to change channel coding during retransmission phase.
EDGE, EGPRS: These two systems apply net rate link adaptation (select channel coding and modulation alphabet) per mobile terminal. No protocol exists yet. However, the structure and protocol is based on the GPRS structure and a similar protocol will be utilised. Extensive simulation studies have been performed on the system throughput and can be found in [2].
The problem with changing channel coding during retransmissions is solved by doing re-segmentation. However, the frame structure used in these systems is not suited for a TDD system.
DVB, DAB: Digital Video/Audio Broadcasting uses different code rates and modulation alphabets to be able to extend their coverage regions and to enable the possibility for an broadcaster to select suitable parameters so that both data and the ordinary program can be sent on the allocated bandwidth, see [3]. In the pure broadcast scenario no uplink signalling exists. Recently, an ACTS program called MEMO has been developed for individual services; the ordinary GSM network is used for the uplink signalling. In this case downlink link adaptation is possible. Still no protocol that enables this signalling exists.
IEEE 802.11: A new physical layer standard is now developed for 5 GHz operation, see [4]. The standard is not fixed yet and the system will apply some sort of link adaptation. The proposed solution is assuming that the physical layer is totally independent from the IEEE 802.11 MAC layer. To enable this a convergence layer, called PHY PLCP (Physical Layer Convergence Protocol), is put in between, where primitives are used through SAPs (Service Access Point) to instruct the physical layer to react.
The selected link parameters are performed by the sending unit, i.e. in the downlink the BS selects the parameters and in the uplink the MT selects the parameters. Both BS and MT are making measurements before selecting PHY (PHYsical layer) parameters, e.g. RSSI measurements.
The access scheme is based on CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). This implies that one MAC frame (in IEEE 802.11 this is equal to a MPDU (MAC Protocol Data Unit)) is transmitted between two peer entities only, i.e. the MAC frame is only between a BS (centrally controlled system) and one MT, or the MAC frame is only between two MTs (Ad-hoc system). The duration of the MAC frame depends on the selected PHY parameter In case of a more robust PHY mode, the length of the PHY frame becomes longer due to higher FEC protection.
This is a gross rate adaptation approach which is not able to consider QoS and fairness between users, i.e. since the transmitting unit is selecting the PHY parameters (used capacity), a user may select a parameter corresponding to a robust PHY mode resulting in larger capacity utilisation even though it is not necessary.
In the current version of the IEEE 802.11 proposal for 5 GHz, measurements needed for the selection of PHY parameters has to be performed by both the BS and the MT.
An object of the present invention is to provide a spectrum efficient radio link adaptation method and frame structure for a TDMA/TDD radio communication system.
This object is achieved in accordance with the attached claims.
Briefly, the present invention uses the BCCH (Broadcast Control CHannel) to adapt the radio cell to prevailing radio conditions. This provides a very efficient method, since a common physical layer parameter indicator may be used for all radio links. An efficient and more flexible embodiment uses a common physical layer parameter indicator to adapt the uplinks of the radio cell, while the downlinks are individually adapted using physical layer parameter indicators in the ACH (Announcement and assignment CHannel). It is also possible to let the BCCH indicate the physical layer parameters to be used for decoding of the ACH.